[ui] Fix device require() path.

ui.lua

@@ -1,4 +1,4 @@
-local BASE = (...):gsub('ui', '')
+local BASE = (...):gsub('ui$', '')
 
 local Widget = require(BASE..'widget')
 local Layout = require(BASE..'layout')
@@ -72,7 +72,7 @@ function Ui.new(args)
     local self = setmetatable(args, Ui)
     assert(#self == 1, "Ui.new() must have exactly one root widget.")
 
-    self.device = self.device or require('device.love').new()
+    self.device = self.device or require(BASE..'device.love').new()
     self.x = self.x or 0
     self.y = self.y or 0
     self.pointerActive = true